1. Field of the Invention
The present invention generally relates to a voltage control oscillator (VCO), and more particular, to a VCO including a back-gate coupling circuit.
2. Description of Related Art
Along with the rapid progress of radio frequency technology (RF technology), the demand on the RF products with high performance and extreme-low cost is increased. Herein the phase noise of an OCR is a key factor affecting the quality of transmitting and receiving information and the reliability of an RF product. Therefore, it has become the most important research topic to develop a VCO with low power-consumption and low phase noise.
In terms of the differential LC-tank VCO which has been developed recently, it is classified into three types, namely cross-coupling VCO, balanced VCO and Transformer-based VCO.
(1) Cross-coupling VCO, shown as FIGS. 1A-1C.
FIG. 1A is a schematic circuit drawing of a conventional transconductance coupling LC-tank VCO with NMOS transistor architecture, wherein inductors and capacitors form a resonant cavity and the resonant cavity is coupled by two NMOS transistors to achieve the differential output effect.
FIG. 1B is a schematic circuit drawing of a conventional transconductance coupling LC-tank VCO with PMOS transistor architecture, wherein the VCO circuit is coupled by two PMOS transistors to achieve the differential output effect.
FIG. 1C is a schematic circuit drawing of a conventional complementary transconductance coupling LC-tank VCO, wherein the VCO circuit is coupled by two PMOS transistors and two NMOS transistors to achieve the differential output effect.
(2) Balanced VCO, shown as FIG. 2. FIG. 2 is a schematic circuit drawing of a conventional balanced VCO, wherein a pair of half-circuits in the oscillator circuit is symmetrical to each other so as to achieve the differential output effect.
(3) Transformer-based VCO, shown as FIG. 3. FIG. 3 is a schematic circuit drawing of a conventional transformer-based VCO, wherein the coupling within a transformer of the oscillator circuit is used to produce the differential output.
In order to improve the phase noise of the above-mentioned conventional VCOs, the following schemes are usually utilized: (1) by improving the inductors or capacitors, for example, octagon inductors are preferably used to reduce the phase noise; (2) by choosing low-noise PMOS transistors as active devices, the phase noise thereof is improved; (3) by employing the inductors in transformer form, the output signal amplitude is enhanced; and (4) by disposing additional capacitors and inductors at the places where a second-harmonic occurs, the phase noise thereof is filtered and reduced.
Although the above-mentioned conventional schemes can improve the phase noise of a VCO, but the schemes also bring the new problems to the VCO that the circuit would occupy more area on a chip, increase power consumption and increase the difficulty of optimizing characteristics.